1. Field of the Invention
The present invention relates to ultrasonic coordinate input apparatuses and ultrasonic coordinate input methods.
2. Description of the Related Art
Coordinate input apparatuses utilizing ultrasonic wave have already been developed. FIG. 1 illustrates an ultrasonic coordinate input apparatus disclosed in Japanese Unexamined Patent Publication No. 2002-373053 (hereinafter referred to as Patent Document 1). FIG. 1 shows the back face 16 of the screen of a terminal 1 in a closed state. In the structure illustrated in FIG. 1, an ultrasonic receiver 2A and an ultrasonic receiver 2B receive ultrasonic wave generated from a pen 4. A paper sheet 6 is placed on the back face 16, and writing is carried out on the paper sheet 6 with the pen 4. A paper holder 18 is to secure the paper sheet 6 to the back face 16.
The operation of the conventional ultrasonic coordinate input apparatus will now be described. When the paper sheet 6 placed on the back face 16 is pressed with the pen 4 or a picture is drawn on the paper sheet 6 with the pen 4, ultrasonic wave and infrared rays are generated from the top end of the pen 4. The generated ultrasonic wave is detected by the ultrasonic receivers 2A and 2B, and the generated infrared rays are detected by an infrared PD 3 that is located in a predetermined position on the back face 16.
The time differences T1 and T2 between the detection of the infrared rays and the detection of the ultrasonic wave are then measured, and the position coordinates of the pen 4 are calculated from the time differences T1 and T2. This process is repeated at regular intervals, and the indicating position of the pen 4 or the location (coordinates) of a picture is read in every time this process is carried out. In this manner, the coordinates of the top end of the pen 4 can be detected every time a position is indicated or a picture is drawn with the pen 4 on the paper sheet 6 placed on the back face 16, with the screen of the terminal 1 being closed and the back face 16 facing the front.
Referring now to FIG. 2, another conventional ultrasonic coordinate input apparatus will be described. FIG. 2 is a block diagram illustrating the structure of the conventional ultrasonic coordinate input apparatus. Like the apparatus disclosed in Patent Document 1, the ultrasonic coordinate input apparatus 20 utilizes infrared rays as synchronization signals of ultrasonic outputs. As shown in FIG. 2, the ultrasonic coordinate input apparatus 20 includes a transmitter 30 and a receiver 40. Reference numeral 50 indicates a host computer such as a personal computer.
The transmitter 30 takes a form of a pen that is the same as the pen 4 shown in FIG. 1, for example. This transmitter 30 includes a power supply 31, a main processor 32, an infrared emitter circuit 33, and an ultrasonic generator circuit 34. The power supply 31 supplies power to the entire transmitter 30. The main processor 32 controls the entire transmitter 30 to drive the infrared emitter circuit 33 and the ultrasonic generator circuit 34. The infrared emitter circuit 33 emits infrared rays. The ultrasonic generator circuit 24 generates ultrasonic wave.
The receiver 40 is to receive ultrasonic wave generated from the transmitter 30. This receiver 40 includes an infrared receiver circuit 41, an ultrasonic receiver circuit 42, a main processor 43, a power supply 44, and a host interface circuit 45. The infrared receiver circuit 41 receives infrared rays emitted from the transmitter 30. The ultrasonic receiver circuits 42 receive ultrasonic wave generated from the transmitter 30. Although not shown in FIG. 2, another ultrasonic receiver circuit 42 is provided in the receiver 40, which is, there are two ultrasonic receiver circuits 42 in the receiver 40, like the ultrasonic coordinate input apparatus of Patent Document 1 having two ultrasonic receivers. The power supply 44 supplies power to the entire receiver 40.
Based on signals transmitted from the infrared receiver circuit 41 to the ultrasonic receiver circuit 42, the main processor 43 measures the time difference between the detection of infrared rays and the detection of ultrasonic wave. The main processor 43 then calculates the position coordinates of the transmitter 30 from the time difference, and outputs the location coordinates to the host computer 50 through the host interface circuit 45. In this manner, the location of the transmitter 30 is detected.
Japanese Unexamined Patent Publication No. 11-15592 (hereinafter referred to as Patent Document 2) discloses yet another conventional apparatus. In a position input apparatus disclosed in Patent Document 2, a vibratory pen is connected to a receiving unit with a cable. Power is supplied to the pen from the outside through the cable, while synchronization signals are transmitted from the pen to the receiving unit.
Japanese Unexamined Patent Publication No. 10-111117 (hereinafter referred to as Patent Document 3) discloses still another conventional apparatus. An ultrasonic digitizer disclosed in Patent Document 3 has a frequency switcher provided in the ultrasonic generator so as to select a frequency among different preset frequencies. With this structure, measurement can be carried out with high accuracy. Japanese Unexamined Patent Publication No. 2000-132682 (hereinafter referred to as Patent Document 4) discloses yet another conventional apparatus. In the apparatus disclosed in Patent Document 4, a vibratory pen is connected to a control circuit with a cable, and power is supplied to the pen from the control circuit through the cable.
However, there are problems with the ultrasonic coordinate input apparatus disclosed in Patent Document 1 and the conventional ultrasonic coordinate input apparatus shown in FIG. 2. In those apparatuses, infrared rays are used as trigger signals for notifying the receiver that ultrasonic wave is outputted from the transmitter. As infrared rays are easily affected by disturbing light such as sunlight, erroneous trigger signal detection is often caused on the receiver end. Also, with infrared rays, an infrared emitter circuit is required for the transmitter, and an infrared receiver circuit is required for the receiver. As a result, a large space for the circuits is needed, and the production cost becomes high.
So as to solve the above problems, Patent Document 2 discloses a technique of connecting a transmitter and a receiver with a cable so that trigger signals can be transmitted. However, the same amount of ultrasonic wave is outputted no matter where the transmitter is located. As a result, electric power is wasted.
In the ultrasonic coordinate input apparatus shown in FIG. 2, a battery is normally used for the power supply 31 of the transmitter 30. However, in a case where the battery voltage becomes lower than the operating voltage, the ultrasonic coordinate input apparatus 20 may become unusable.
So as to solve this problem, Patent Document 2 and Patent Document 4 each discloses a technique of not employing a power supply in the transmitter, and utilizing the power supply of the receiver to output ultrasonic wave from the transmitter. However, as the same amount of ultrasonic wave is outputted no manner where the transmitter is located, electric power is wasted, and control operations cannot be performed in accordance with the location of the transmitter.